Cardiac arrythmias, and atrial fibrillation in particular, persist as common and dangerous medical ailments, especially in the aging population. In patients with normal sinus rhythm, the heart, which is comprised of atrial, ventricular, and excitatory conduction tissue, is electrically excited to beat in a synchronous, patterned fashion. In patients with cardiac arrythmias, abnormal regions of cardiac tissue do not follow the synchronous beating cycle associated with normally conductive tissue as in patients with normal sinus rhythm. Instead, the abnormal regions of cardiac tissue aberrantly conduct to adjacent tissue, thereby disrupting the cardiac cycle into an asynchronous cardiac rhythm. Such abnormal conduction has been previously known to occur at various regions of the heart, such as, for example, in the region of the sino-atrial (SA) node, along the conduction pathways of the atrioventricular (AV) node and the Bundle of His, or in the cardiac muscle tissue forming the walls of the ventricular and atrial cardiac chambers.
Cardiac arrhythmias, including atrial arrhythmias, may be of a multiwavelet reentrant type, characterized by multiple asynchronous loops of electrical impulses that are scattered about the atrial chamber and are often self propagating. Alternatively, or in addition to the multiwavelet reentrant type, cardiac arrhythmias may also have a focal origin, such as when an isolated region of tissue in an atrium fires autonomously in a rapid, repetitive fashion. Ventricular tachycardia (V-tach or VT) is a tachycardia, or fast heart rhythm that originates in one of the ventricles of the heart. This is a potentially life-threatening arrhythmia because it may lead to ventricular fibrillation and sudden death.
Diagnosis and treatment of cardiac arrythmias include mapping the electrical properties of heart tissue, especially the endocardium and the heart volume, and selectively ablating cardiac tissue by application of energy. Such ablation can cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. Various energy delivery modalities have been disclosed for forming lesions, and include use of microwave, laser and more commonly, radiofrequency energies to create conduction blocks along the cardiac tissue wall. In a two-step procedure—mapping followed by ablation—electrical activity at points within the heart is typically sensed and measured by advancing a catheter containing one or more electrical sensors (or electrodes) into the heart, and acquiring data at a multiplicity of points. These data are then utilized to select the endocardial target areas at which ablation is to be performed.
Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity. In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral artery, and then guided into the chamber of the heart of concern. A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the skin of the patient or by means of a second catheter that is positioned in or near the heart. RF (radio frequency) current is applied to the tip electrode of the ablating catheter, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, possibly above 60.degree. C., a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker resulting in blood coagulation on the electrode surface. Because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. If the impedance increases sufficiently, an impedance rise occurs and the catheter must be removed from the body and the tip electrode cleaned.
In a typical application of RF current to the endocardium, circulating blood provides some cooling of the ablation electrode. However, there is typically a stagnant area between the electrode and tissue which is susceptible to the formation of dehydrated proteins and coagulum. As power and/or ablation time increases, the likelihood of an impedance rise also increases. As a result of this process, there has been a natural upper bound on the amount of energy which can be delivered to cardiac tissue and therefore the size of RF lesions. Historically, RF lesions have been hemispherical in shape with maximum lesion dimensions of approximately 6 mm in diameter and 3 to 5 mm in depth.
It is desirable to reduce or eliminate impedance rises and, for certain cardiac arrhythmias, to create larger lesions. One method for accomplishing this is to irrigate the ablation electrode, e.g., with physiologic saline at room temperature, to actively cool the ablation electrode instead of relying on the more passive physiological cooling of the blood. Because the strength of the RF current is no longer limited by the interface temperature, current can be increased. This results in lesions which tend to be larger and more spherical, usually measuring about 10 to 12 mm.
The effectiveness of irrigating the ablation electrode is dependent upon the distribution of flow within the electrode structure and the rate of irrigation flow through the tip. Effectiveness is achieved by reducing the overall electrode temperature and eliminating hot spots in the ablation electrode which can initiate coagulum formation.
More channels and higher flows are more effective in reducing overall temperature and temperature variations, i.e., hot spots. However, the coolant flow rate should be balanced against the amount of fluid that can be injected into a patient and the increased clinical load required to monitor and possibly refill the injection devices during a procedure. In addition to irrigation flow during ablation, a maintenance flow, typically at a lower flow rate, is required throughout the procedure to prevent backflow of blood flow into the coolant passages. Thus reducing coolant flow by utilizing it as efficiently as possible is a desirable design objective.
The arrangement of conventional internal catheter components such as irrigation lumens, location sensor and related electrical leads is limited by available cross-sectional area of the tip electrode. The limiting direction is typically in the radial direction emanating from the axial centerline of the tip electrode radiating to the outer periphery. Conventional irrigation tubings or the through-passage formed in the tip electrode receiving an irrigation tubing has a circular cross-section and is therefore limited in size by this radial dimension. Furthermore it is generally desirable to have the largest possible fluid lumen in order to minimize hydraulic resistance/pressure drop over the length of the catheter shaft. These factors can often result in a design using either a smaller-than-desired fluid lumen, or a two-piece tubing possessing a larger diameter in the catheter shaft and a smaller diameter coupler at the tip electrode. The inclusion of the coupler results in an additional adhesive bond joint which contributes to a higher risk of fluid leaks.
Moreover, conventional irrigated ablation tip electrodes are designed as solid monolithic structures with internal fluid paths and fluid ports where the internal fluid paths are much longer, if not two, three, or four times longer, than the size of the fluid port. Where fluid flow along the length of the catheter shaft is assumed to be laminar, Poiseuille's law states that pressure drop over a distance is proportional to the flow rate multiplied by the hydraulic resistance, where hydraulic resistant relates fluid viscosity and conduit geometry. Because of the temperature of the irrigating fluid and consequently the high viscosity of the fluid relative to the port diameter, and the length of the irrigation tubing, a significant amount of energy is required to pump the fluid to the tip electrode.
Conventional irrigated ablation tip electrodes also typically have a much greater total fluid output area compared to fluid input area where the fluid output area is a two, three or four multiple of the fluid input area. As such, the flow of irrigation fluid out of the outlet fluid ports is primarily governed by the inertia of the fluid. Applying the law of conservation where the flow of the fluid into the electrode equals the flow of fluid out of the electrode, a significant amount of energy is used not only to pump the fluid to the tip electrode, but to provide the fluid with a desirable exit velocity from the electrode.
Another concern with conventional irrigated ablation tip electrodes is the axially variability of fluid mass flow rate through the tip electrode. Fluid entering a proximal end of a tip electrode chamber carries momentum in the axial direction such that more fluid tends to exit the fluid ports at the distal end compared to fluid ports on the radial side of the tip electrode. Such uneven distribution of fluid can cause undesirable “hot spots” which can compromise the size and quality of the lesions and require interruption of the ablation procedure so that coagulation can be removed from the tip electrode.
Ablation electrodes using a porous material structure can provide efficient coolant flow. The porous material in which tiny particles are sintered together to form a metallic structure provides a multiplicity of interconnected passages which allow for efficient cooling of an electrode structure. However, because the particles are sintered together, there can be concerns with particles detaching from the electrode and entering the bloodstream.
Accordingly, it is desirable that a catheter be adapted for mapping and ablation with improved irrigation fluid flow by means of more efficient use of the space in the tip electrode that avoids the introduction of additional bonding joints. It is desirable that an irrigated tip electrode use provides an internal fluid path that has a better consideration and utilization of inherent fluid dynamics for improved fluid flow and cooling of the tip electrode.